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Chapter 12: The Somatic Sensory System

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Somatic Sensation
Enables our body to feel, to ache, to sense hot or chill, and to know what its parts are doing. It is very sensitive to stimuli
Somatic Sensory System’s Four Senses
We can think of this system as a group of at least four senses rather than a single one: the senses of touch, temperature, pain, and body position
Two Major Types of Skin
Two Major Types of Skin
The sensation of touch begins at the skin. The two major types of skin are called *hairy* and *glabrous*(hairless)(palm of our hand)
Epidermis
Epidermis
The outer layer of skin
Dermis
Dermis
The inner layer of skin
Mechanoreceptors
most somatic sensory receptors are mechanoreceptors, sensitive to physical distortion , feel contact with the skin, pressure in the heart and blood vessels, stretching of bladder.

Pacinian Corpuscle
Pacinian Corpuscle
The largest and best studied mechanoreceptor which lies deep in the dermis. Each human hand has 2500 Pacinian corpuscles, with the highest densities in the fingers
Ruffini's Endings
Ruffini’s Endings
Mechanoreceptors that are found in both hairy and glabrous skin and are slightly smaller than Pacinian corpuscles
Meissner's Corpuscles
Meissner’s Corpuscles
Mechanoreceptors that are about 1 tenth the size of Pacinian corpuscles and are located in the ridges of glabrous skin(the raised parts of your fingerprints, for example)
Merkel's Disks
Merkel’s Disks
Mechanoreceptors that are located in the epidermis which each consist of a nerve terminal and a flattened, non-neural epithelial cell (the Merkel cell)
Krause End Bulbs
Mechanoreceptors which lie in the border regions of dry skin and mucous membrane(around the lips and genitals, for example), and have nerve terminals look like knotted balls of string
Receptive Field of Human Sensory Receptors
Receptive Field of Human Sensory Receptors
Meissner’s corpuscles and Merkel’s disks had small receptive fields, only a few mellimeters wide, while Pacinian corpuscles and Ruffini’s endings had large receptive fields that could cover an entire finger or half the palm
Rapidly Adapting Receptors
Rapidly Adapting Receptors
Mechanoreceptors, such as Meissner’s and Pacinian corpuscles, that tend to respond quickly at first to stimulus but them stop firing even through the stimulus continues
Slowly Adapting Receptors
Slowly Adapting Receptors
Mechanoreceptors, such as Merkel’s disks and Ruffini’s endings, that generate a more sustained response during a long stimulus
Follicles
Follicles
Hairs grow from *follicles* embedded in the skin; each follicle is richly innervated by free nerve endings – the terminations of single axons – that either wrap around the follicle or run parallel to it.
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Frequency Sensitivity of Types of Mechanoreceptors ( not important)
Frequency Sensitivity of Types of Mechanoreceptors ( not important)
Different mechanical sensitivities of mechanoreceptors mediate different sensations . Pacinian receptors are more sensitive to vibrations of about 200-300 Hz, while Meissner’s corpuscles respond best to around 50 Hz
Selectivity of Mechanoreceptor
The selectivity of a mechanoreceptive axon depends primarily on the structure of its special endings.
Vibration and Pacinian Corpuscle ( not important)
Vibration and Pacinian Corpuscle ( not important)
When the capsule like ending of a Pacinian corpuscle is compressed, energy is transferred to the nerve terminal, its membrane is deformed, and mechanosensitive channels open, stimulating an action potential. When the pressure is released, the events reverse themselves; the terminal depolarizes again and may fire another action potential
Mechanosensitive Ion Channels
Mechanosensitive Ion Channels
The mechanoreceptors of the skin all have unmyelinated axon terminals, and the membranes of these axons have *mechanosensitive ion channels* that convert mechanical force into a change of ionic current
Types of Sensitivity of Mechanosensitive Ion Channels
Types of Sensitivity of Mechanosensitive Ion Channels
This is the explanation of the image in the previous card
Two-Point Discrimination on the Body Surface
Two-Point Discrimination on the Body Surface
Our ability to discriminate the detailed features of a stimulus varies tremendously across the body
( to test the spatial resolution)
reason for the difference
1.receptive field density
2.receptive field size
3.computing power
4.special neural mechanisms
Primary Afferent Axons
Primary Afferent Axons
Axons bringing information from the somatic sensory receptors to the spinal cord or brain stem are the *primary afferent axons* of the somatic sensory system. The skin is richly innervated by these axons and make their way to the CNS
Primary Afferent Axons Entering the Spinal Cord
Primary Afferent Axons Entering the Spinal Cord
The primary afferent axons enter the spinal cord through the dorsal roots; their cell bodies lie in the dorsal root ganglia
Various Sizes of Primary Afferent Axons
Various Sizes of Primary Afferent Axons
Primary afferent axons sizes correlate with the type of sensory receptor to which they are attached.
Axons from skin sensory receptors are usually designated
Aα( group 1), -proprioceptors of skeletal muscle
Aβ,( group II) – mediates touch sensations (mechno receptor of skin)
Aδ ( group III), -pain and temperature
C ( group IV)- fibers only mediate pain, itch and temperature
the groups mean axon from muscules
Group C(or IV) axons are, by definition, unmyelinated axons, while all the rest are myelinated
The Spinal Cord
Most peripheral nerves communicate with the CNS via the spinal cord, which is encased in the bony vertebral column
Organization of the Spinal Cord
Organization of the Spinal Cord

The arrangement of the paired dorsal and ventral roots is repeated 30 times down the length of the human spinal cord.

Each spinal cord passes through a notch between the vertebrae and there are as many spinal nerves as their are notches.

The 30 *spinal segments* are divided into four groups

Segmental Organization of the Spinal Cord
Segmental Organization of the Spinal Cord

Each segment of the spinal cord is named after the vertebra adjacent to where the nerves originate;

cervical(C)1-8,
thoracic(T) 1-12,
lumbar(L) 1-5, and
sacral(S) 1-5

Dermatone
Dermatone
The segmental organization of spinal nerves and the sensory innervation of the skin are related.
The area of skin innervated by the right and left dorsal roots of a single spinal segment is called the *dermatome*;
thus, there is a one-to-one correspondence between dermatomes and spinal segments.
ex) shingles
Organization of Dermatone
Organization of Dermatone
When mapped, the dermatomes delineate a set of bands on the body surface
Organization of Dermatone with Human on All Fours
Organization of Dermatone with Human on All Fours
Cauda Equina
Cauda Equina
The bundles of spinal nerves streaming down within the lumbar and sacral vertebra. These course down the spinal column within a sack of dura with cerebrospinal fluid(CSF)
Sensory Organization of the Spinal Cord
Sensory Organization of the Spinal Cord

The spinal cord is composed of an inner core of gray matter, surrounded by a thick covering of white matter tracts that are often called *columns*.

Each half of the spinal gray matter is divided into a *dorsal horn*, an *intermediate zone*, and a *ventral horn*

Second-Order Sensory Neurons
Second-Order Sensory Neurons

The neurons that receive sensory input from primary afferents are called *second-order sensory neurons*.

Most of the second-order sensory neurons of the spinal cord lie within the dorsal horn

Dorsal Column Medial Lemniscal Pathway
Dorsal Column Medial Lemniscal Pathway

The pathway serving for touch.

1.The ascending branch of the large sensory axons(Aβ) enters the ipsilateral *dorsal column* of the spinal cord(carry tactile sensation and limb position toward brain) and terminate in the *dorsal column nuclei*.

2.The axons of the dorsal column nuclei decussate(cross-over) and ascend within the *medial lemniscus*, whose axons then synapse upon neurons of the *ventral posterior (VP) nucleus* of the thalumus.

3.Thalamic neurons of the VP nucleus then project to specific regions of the *primary somatosensory cortex*, or *S1*

Somatosensory Cortex
Somatosensory Cortex
Most of the cortex concerned with the somatic sensory system is located in the parietal lobe
Primary Somatosensory Cortex (S1 = 3b)
Primary Somatosensory Cortex (S1 = 3b)
Brodmann’s area *3b* is regarded as the primary somatosensory cortex(*S1*), and it lies on the postcentral gyrus(right behind central sulcus). So S1 contains area 3b as well as 3a, 1, and 2.
Other Cortical Areas that Process Somatic Sensory Information
Other Cortical Areas that Process Somatic Sensory Information
Other cortical areas that process somatic sensory information include areas 3a, 1, and 2 on the *postcentral gyrus*, and areas 5 and 7 on the adjacent *posterior parietal cortex*
Area 3b as the Primary Somatic Sensory Cortex
Area 3b as the Primary Somatic Sensory Cortex
Area 3b
(1) receives dense inputs from the VP nucleus of the thalumus;
(2) its neurons are very responsive to the somatosensory stimuli(but not to other stimuli);
(3) lesions here impair somatic sensation; and
(4) when electrically stimulated, it evokes somatic sensory experiences
Function of Area 3a
Function of Area 3a
Area 3a is concerned with the sense of body position(joints and muscles)(proprioceptive information) rather than touch
Function of Areas 1 and 2
Function of Areas 1 and 2
Areas 1 receive texture inputs from area 3b.
Area 2: recieves size and shape info input from 3b
Lesions in Areas 1 and 2
Lesions in Areas 1 and 2
Small lesions in area 1 or 2 produce predictable deficiencies in discrimination of texture, size, and shape
Columnar Organization of S1's Area 3b
Columnar Organization of S1’s Area 3b
S1 neurons with similar inputs and responses are stacked vertically into columns that extend across the cortical layers
Somatotopy

The receptive fields of many S1 neurons produce an orderly map of the body on the cortex.

The mapping of the body’s surface sensations onto a structure in the brain is called *somatotopy*

Somatotopic Maps
Somatotopic Maps
Somatotopic maps roughly resemble a body with its legs and feet at the top of the postcentral grus and its head at the opposite, lower end of the gyrus
Homunculus
Homunculus

A somatotopic map is sometimes called a *homunculus*.

This is a map of the relative size of the cortex devoted to each body part and it correlated with the density of sensory input received form that part.

The importance of an input, and the size of its representation in cortex, are also reflections of how often it is used

S1 of Rodents
S1 of Rodents

The large facial vibrissae(whiskers) of rodents receive a huge share of the territory in S1 and the sensory signals from each vibrissae follicle go to one clearly defined cluster of the S1 neurons;

such clusters are called barrels. The five rows of cortical barrels precisely match the five rows of facial vibrissae

Multiple Somatic Maps
Multiple Somatic Maps
Somatotopy in the cerebral cortex is not limited to a single map. The somatic sensory system has several maps of the body.
Somatotopic Map Plasticity ( last slide for first packet)
Somatotopic Map Plasticity ( last slide for first packet)
Agnosia
The inability to recognize objects even though simple sensory skills seem to be normal which results from damage to the posterior parietal cortex
Nociceptors

Somatic sensation depends strongly on *nociceptors*, the free, branching, unmyelinated nerve endings

sensory process provides signals that trigger pain

Transduction of pain

types of noiceptros : polymodal: response to mechanical thermal and chemical stimuli

Pain

Selective activation of nociceptors can lead to the conscious experience of pain.

Feelings of Sore, aching or throbbing

Difference Between Nociception and Pain
Pain is a feeling or perception. Nociception is the sensory process that provides signals that trigger pain
Painful Stimuli
Tissue damage can result from strong mechanical stimulation, extremes in temperature, oxygen deprivation, and exposure to certain chemicals, among other causes. The membranes of nociceptors contain ion channels that are activated by these types of stimuli
Stimulus and Nociceptors
Simple stretching or bending of the nociceptor membrane activates mechanically gated ion channels that cause the cell to depolarize and generate action potentials
Chemical Stimulus for Pain

Damaged cells at the site of injury can release a number of substances that cause ion channels on nociceptor membranes to open.

Examples of release substances are
1.proteases(enzymes that digest proteins),
2.ATP), K^+, bradykinin, lactic acid,

Itch
Disagreeable sensation that induces desire or reflex to scratch
First and Second Pain
The activation of skin nociceptors produces two distinct perceptions of pain:
1. fast, sharp, first pain followed by a duller, longer lasting second pain.
First pain is caused by the activation of Aẟ fibers;second pain is caused by the activation of C fibers

1st Difference Between Touch and Pain Pathway
They differ with respect to their nerve endings in the skin. The touch pathway is characterized by specialized structures in the skin; the pain pathway has only free nerve endings
2nd Difference Between Touch and Pain Pathway
They differ with respect to the diameter of their axons. The touch pathway is swift, using fat, myelinated Aβ fibers; the pain pathway is slow, using thin, lightly myelinated Aδ fibers and unmyelinated C fibers
3rd Difference Between Touch and Pain Pathway
They differ with respect to their connections in the spinal cord. Branches of Aβ axons terminate in the deep dorsal horn; the Aδ and C fibers branch run within the zone of Lissauer, and termiante within the substantia gelatinosa
Spinothalamic Pain Pathway
Spinothalamic Pain Pathway
Information about pain(as well as temperature) in the body conveyed from the spinal cord to the brain via the *spinothalamic pathway*. The axons of the second-order neurons *immediately decussate* and ascend through the spinothalamic tract running along the ventral surface of the spinal cord
Comparison of Pathways for Touch and Pain
Comparison of Pathways for Touch and Pain
Information about touch ascends *ipsilaterally*, while information about pain(and temperature) ascends *contralaterally*
Comparison of Spinal Cord Deficits Effect on Pathways for Touch and Pain
Comparison of Spinal Cord Deficits Effect on Pathways for Touch and Pain
If half of the spinal cord is damaged, certain deficits of mechanosensitivity occur on the *same side* as the spinal cord damage. Deficits in pain and temperature sensitivity will show up on the side of the body *opposite* the cord damage
The Trigeminal Pain Pathway
Pain(and temperature) information from the face and head takes a path to the thalamus that is analogous to the spinal path called the *trigeminal pain pathway*. The axon of these cells cross and ascend to the thalamus in the *trigeminal lemniscus*
Segregation of Touch and Pain Systems
Some of the axons of the spinothalamic tract and trigeminal lemniscal axons(both pain) terminate in the VP nucleus, just as the medial lemniscus(touch) but the touch and pain systems still remain segregated.
The Cortex and Pain Pathway
From the thalamus, pain and temperature information is projected to various areas of the cerebral cortex
Afferent Regulation of Pain
Light touch can evoke pain via hyperalgesia. However, pain evoked by activity in nociceptors can also be reduced by simultaneous activity in low-threshold mechanoreceptors(Aβ)
Summary of Gate Theory of Pain
Summary of Gate Theory of Pain
Activity in the pain axon alone maximally excites the projection neuron, allowing nociceptive signals to rise to the brain. However, if the large mechanoreceptive axons(touch) fires concurrently, it activates the interneuron and suppresses nociceptive(pain) signals
Projection Neuron of Gate Theory of Pain
Projection Neuron of Gate Theory of Pain
This theory suggests that certain neurons(projection neurons) of the dorsal horns, which project an axon up the spinothalamic tract, are excited by both large-diamter sensory axons(touch axons) and unmylelinated pain axons.
Interneuron of Gate Theory of Pain
Interneuron of Gate Theory of Pain
The projection neurons is also inhibited by an interneuron, and the interneuron is both excited by the large sensory axon and inhibited by the pain axon
Descending Regulation of Pain
Descending Regulation of Pain
Strong emotion can powerfully suppress feelings of pain. One zone of neurons that has been implicated in pain suppression in the midbrain is called the periventricular and *periaqueductal gray matter (PAG)*. Electrical stimulation of the PAG can cause a profound analgesia that has sometimes been exploited clinically
Opiods
Produce profound analgesia when taken systemically
Opiod Receptors
Opiods act by binding tightly and specifically to several types of *opiod receptors* in the brain, and that the brain itself manufactures endogenous morphine-like substances, collectively called *endorphins*
Endorphins
Relatively small proteins, or peptides. Endorphins and their receptors are widely distributed in the CNS, but are concentrated in areas that process or modulate nociceptive information
Endorphin-Containing Neurons
Endorphin-containing neurons in the spinal cord and brain stem prevent the passage of nociceptive signals through the dorsal horn and into higher levels of the brain where the perception of pain is generated
Temperature Sensations
Nonpainful temperature sensations originate from receptors in the skin(and elsewhere), and they depend on the neocortex for their conscious appreciation
Thermoreceptors
Neurons that are exquisitely sensitive to temperature because of specific membrane mechanisms. These receptors contribute to our perception of temperature
Hyperalgesia
Can be a reduced threshold for pain, an increased intensity of painful stimuli, or even spontaneous pain
Spinal Connections of Nociceptive Axons
Spinal Connections of Nociceptive Axons

The small-diameter fibers have their cell bodies in the segmental dorsal root ganglia, and they enter the dorsal horn of the spinal cord.

The fibers branch out immediately, travel a short distance up and down the spinal cord in a region called the *zone of Lissauer*,

and then synapse on cells in the outer part of the dorsal horn in a region known as the *substantia gelatinosa*

Referred Pain
The cross-talk between visceral and cutaneous nociceptors gives rise to the phenomenon of *referred pain*, where visceral nociceptor activation is perceived as a cutaneous sensation

Cite this Chapter 12: The Somatic Sensory System

Chapter 12: The Somatic Sensory System. (2017, Dec 06). Retrieved from https://graduateway.com/chapter-12-the-somatic-sensory-system-essay/

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